The long term objectives of this project are to elucidate the molecular mechanisms of viral membrane fusion proteins and to use this information in the rational design of novel anti-viral agents. The influenza HA remains the best characterized viral fusion protein. When exposed to low pH, HA changes conformation, exposes its fusion peptide, binds to the target membrane, and induces the formation of a 'fusion pore'. Our major hypotheses are (1) that these general principles are used by all viral fusion proteins and (2) that for fusion proteins that function at neutral pH (e.g., the HIV env), an interaction between the viral glycoprotein and its host cell receptor triggers a fusion-inducing conformational change. Our first goal is to test these hypotheses for the Rous sarcoma virus env glycoprotein, a model retroviral fusion protein that functions at neutral pH. Our second goal is to rationally design an inhibitor of the fusion- inducing conformational change in the influenza HA. The specific aims are: 1. to characterize further the fusion mechanism of the influenza HA; 2. to test whether an interaction between the RSV env glycoprotein and its receptor trigger: a fusion-inducing conformational change; 3. to compare subsequent steps in the fusion mechanisms of RSV env and HA; and 4. to pursue our discovery, using a rational design strategy, of a family of benzo- and hydroquinones that prevents the fusion-inducing conformational change in HA. The experimental design is modeled on previous studies of the influenza HA. It employs virological, biochemical, cell, and molecular biological techniques. The project has a high degree of health relatedness. Little is known about how viruses such as paramyxo-, herpes-, and retroviruses fuse with their host cells at neutral pH. The proposed studies on RSV env, performed as a comparative analysis with studies on influenza HA, should help fill this gap. In addition, given the similarities among retroviral env proteins, the work on RSV is highly relevant to the infectious entry process of two devastating human retroviruses, HIV and HTLV. A fundamental premise is that understanding the molecular mechanism of fusion will lead to the development of rational strategies to inhibit the entry of these and other pathogenic viruses. In the case of HA, information on the fusion mechanism is now being coupled with structural information to rationally design fusion inhibitors. This strategy should be generalizable to other enveloped viruses.